Light amplification device and laser processing device

A light amplifier according to an aspect of the present invention includes: a seed light source configured to generate a pulsing seed light; an excitation light source configured to generate excitation light; a light amplifying fiber configured to amplify the seed light by the excitation light and output the amplified light; and a control unit configured to control the seed light source and the excitation light source. The control unit has a mode to control the excitation light's power such that as a set value of a pulse width of the amplified light increases, the amplified light's peak energy increases within a threshold value at a minimum set value of the pulse width.